AJP - Heart and Circulatory Physiology, Vol 249, Issue 2 404-H414, Copyright © 1985 by American Physiological Society
Phentolamine-resistant neurogenic constriction occurs in small arteries at higher frequencies
M. P. Owen, C. Quinn and J. A. Bevan
The effect of the alpha 1, alpha 2-antagonist phentolamine (PTA) on
neuromuscular transmission and exogenous norepinephrine (NE) was assessed
in arteries of diminishing diameter possessing a substantial
adventitiomedial junction adrenergic innervation in the rabbit ear, i.e.,
central ear artery (CEA), unstretched lumen diameter (ULD) approximately
equal to 300 microns; main side branch (MSB) off the CEA (ULD approximately
equal to 150 microns); and terminal branch (TB) off the MSB (ULD
approximately equal to 75 microns). With increasing PTA concentrations,
contractile responses to transmural nerve stimulation (TNS) were decreased
proportionately less in TB than in MSB and CEA. PTA (4 X 10(-6) M, a
competitive antagonist concentration) blocked the tetrodotoxin-sensitive
TNS-induced contractions of CEA segments at 2, 4, and 8 Hz. The response at
8 Hz was reduced at least 98% in MSB and 86% in TB. However, responses to 8
Hz were not abolished in MSB and TB until 2 X 10(-5) and 3 X 10(-5) M PTA,
respectively. PTA (3 X 10(-5) M) possessed nonspecific depressant
properties in addition to its alpha-antagonist properties. We conclude that
the sympathetic nervous system influences tone through alpha-adrenoceptors
in the CEA and in the MSB and TB at lower frequencies. Responses in the MSB
and TB at higher frequencies of nerve stimulation are mediated
predominately through alpha-receptors. If the possibility of a
nonadrenergic transmitter is discounted, the possibilities that the small
PTA-resistant component of the neurogenic response is due to a high
concentration of NE acting on alpha-adrenoceptors and/or a high threshold
site cannot be distinguished.
